Silver halide photographic light-sensitive materials are required to have various characteristics, of which the sensitivity and preservability under high temperature/humidity conditions largely affect the ease of handling of light-sensitive materials for photographing and print-making use.
For example, a light-sensitive material for photographing use is required to have a sensitivity as high as ISO 400 or more in consideration of being used in an inexpensive disposable camera having a lens aperture of F8 to F11 and a shutter speed of about 1/100 sec, and further used under severe outdoor exposure conditions such as in the beach, poolside and rainy weather.
Also in the color print-making field, with the recent increase in the number of mini-photofinisher labs, there has been an increasing demand for high-speed light-sensitive materials which enables to make prints in a shorter time suitable for over-the-counter processing and which has an excellent preservability; i.e., whose characteristics are stable over a long period of time even under high humidity conditions.
In order to obtain a high sensitivity, attempts have conventionally been made to raise both light-absorbability and developability of silver halide. For example, a silver iodobromide light-sensitive material uses core/shell-type silver halide grains in which the silver iodide content of the core is higher than that of the shell. This technical means, however, has the problem that as the iodide content of the shell becomes reduced, it becomes harder for the light-sensitive material to obtain an intended color sensitivity for its inherent high sensitivity, or the sensitivity becomes deteriorated under high temperature/humidity conditions.
The color sensitivity can be improved by increasing the iodide content of the surface of silver halide grains. The conventional techniques for increasing the silver iodide content of the surface of silver halide grains include a technique for increasing the silver iodide content of the shell of an internal high iodide content-type core/shell grains and the technique for the internal low iodide content-type core/shell grains described in Japanese Patent Publication Open to Public Inspection (hereinafter referred to as JP O.P.I.) No. 284848/1989.
In the core/shell grains, however, if the silver iodide content of the shell is increased, the chemically sensitized nuclei formed by chemical sensitization are dispersed to cause the grains to be considerably desensitized and further the developability to be largely reduced.
JP O.P.I. No. 106745/1988 discloses a technique producing a low iodide-content shell to cover silver grains with a layer having a thickness of about 50 .ANG. containing silver iodide of 5 mole % or more. Even this method, however, has not attained the solution of the problems of deterioration of the initial developability and dispersion of the chemically sensitized nuclei because the high iodide content layer on the grains surface has a thickness of more than 10 lattices.
JP O.P.I. Nos. 51627/1973 and 77443/1984 disclose a method of adding a water-soluble iodide to a silver iodobromide emulsion for the purpose of improving the color sensitivity.
The above method is useful for increasing the adsorption of a sensitizing dye to the surface of silver halide grains to control the spectral sensitivity distribution thereof or for reducing the desorption of the sensitizing dye under high temperature/humidity conditions, but has the disadvantage that if the water-soluble iodide is added until the adsorption of the sensitizing dye is sufficiently raised, then the sensitivity of the silver halide is lowered. In this method, probably because the adsorption reaction of the iodide ion to the surface of silver halide grains is very rapid and the adsorption is neither uniform nor stable, there are cases where the sensitivity of the resulting silver halide grains changes with time even when stored in a refrigerator, and thus it is difficult to produce a light-sensitive material product having a stable quality.
On the other hand, known as a means for increasing the sensitivity and improving the preservability of a silver chlorobromide emulsion is the method of adding a water-soluble bromide or a water-soluble iodide to the emulsion as described in JP O.P.I. Nos. 96331/11982 and 5238/11984.
However, this method, when a water-soluble bromide alone is added, requires the addition of the bromide in an amount of 5 to 50 mole % per mole of silver, which, in processing, causes an adverse effect such as sensitivity drop or contrast reduction due to the flow-out of the bromide ion in the processing solution. Where a water-soluble bromide and a water-soluble iodide are used in combination, probably because the adsorption reaction of the iodide ion is not uniform or unstable, very conspicuous changes in the photographic characteristics such as sensitivity drop, contrast reduction and increase in fog occur during the period between the emulsion preparation and the emulsion coating, and therefore it is difficult to produce a photographic light-sensitive material having a stable quality.
Thus, the conventional techniques to solve the problems of the color sensitivity drop and preservability deterioration particularly under high temperature/humidity conditions that occur in high sensitization of silver halide emulsions having various compositions are terribly insufficient.